docs: def arg(self, name, help, type=None, choices=None, metavar=None,
              nargs=None):
          """Describe a positional argument"""
          kw = dict(help=help, type=type, choices=choices, metavar=metavar,
                    nargs=nargs)
          default = self.argdict[name]
          if default is not NODEFAULT:
              kw['nargs'] = nargs or '?'
              kw['default'] = default
              kw['help'] = kw['help'] + ' [default: %s]' % repr(default)
          self._add(name, name, **kw)